HiSET Science: Stars and Other Objects in Space

Stellar Observation
The observation of stars relates to one of three stellar properties: position, brightness, and spectra. Positional
stellar observation is principally performed through study of the positions of stars on multiple photographic plates. Historically, this type of analysis was done through measurement of the angular positions of the stars in the sky. Parallax of a star is its apparent shift in position due to the revolution of the Earth about the Sun; this property can be used to establish the distance to a star. Observation of the brightness of a star involves the categorization of stars according to their magnitudes. There is a fixed intensity ratio between each of the six magnitudes. Since stars emit light over a range of wavelengths, viewing a star at different wavelengths can give an indication of its temperature. The analysis of stars' spectra provides information about the temperatures of stars—the higher a star's temperature, the more ionized the gas in its outer layer. A star's spectrum also relates to its chemical composition.

Binary Star
Binary star systems, of which about fifty percent of the stars in the sky are members, consist of two stars that orbit each other. The orbits of and distances between members of a binary system vary. A visual binary is a pair of stars that can be visually observed. Positional measurements of a visual binary reveal the orbital paths of the two stars. Astronomers can identify astrometric binaries through long-term observation of a visible star—if the star appears to wobble, it may be inferred that it is orbiting a companion star that is not visible. An eclipsing binary can be identified through observation of the brightness of a star. Variations in the visual brightness of a star can occur when one star in a binary system passes in front of the other. Sometimes, variations in the spectral lines of a star occur because it is in a binary system. This type of binary is a spectroscopic binary.

Hertzsprung-Russell Diagram
The Hertzsprung-Russell (H-R) diagram was developed to explore the relationships between the luminosities and spectral qualities of stars. This diagram involves plotting these qualities on a graph, with absolute magnitude (luminosity) on the vertical and spectral class on the horizontal.
Plotting a number of stars on the H-R diagram demonstrates that stars fall into narrowly defined regions, which correspond to stages in stellar evolution. Most stars are situated in a diagonal strip that runs from the top-left (high temperature, high luminosity) to the lower-right (low temperature, low luminosity). This diagonal line shows stars in the main sequence of evolution (often called dwarfs). Stars that fall above this line on the diagram (low temperature, high luminosity) are believed to be much larger than the stars on the main sequence (because their high luminosities are not due to higher temperatures than main sequence stars); they are termed giants and supergiants.
Stars below the main sequence (high temperature, low luminosity) are called white dwarfs. The H-R diagram is useful in calculating distances to stars.

Stellar Evolution
The life cycle of a star is closely related to its mass—low-mass stars become white dwarfs, while high-mass stars become supernovae. A star is born when a protostar is formed from a collapsing interstellar cloud. The temperature at the center of the protostar rises, allowing nucleosynthesis to begin. Nucleosynthesis, or hydrogen-burning through fusion, entails a release of energy. Eventually, the star runs out of fuel (hydrogen). If the star is relatively low mass, the disruption of hydrostatic equilibrium allows the star to contract due to gravity. This raises the temperature just outside the core to a point at which nucleosynthesis and a different kind of fusion (with helium as fuel) that produces a carbon nucleus can occur. The star swells with greater energy, becoming a red giant. Once this phase is over, gravity becomes active again, shrinking the star until the degeneracy pressure of electrons begins to operate, creating a white dwarf that will eventually burn out. If the star has a high mass, the depletion of hydrogen creates a supernova.

Supernova
When a star on the main sequence runs out of hydrogen fuel, it begins to burn helium (the by-product of nucleosynthesis). Once helium-burning is complete in a massive star, the mass causes the core temperature to rise, enabling the fusion of carbon, then silicon, and a succession of other atomic nuclei, each of which takes place in a new shell further out of the core. When the fusion cycle reaches iron (which cannot serve as fuel for a nuclear reaction), an iron core begins to form, which accumulates over time. Eventually, the temperature and pressure in the core become high enough for electrons to interact with protons in the iron nuclei to produce neutrons. In a matter of moments, this reaction is complete. The core falls and collides with the star's outer envelope, causing a massive explosion (a supernova). This continues until the neutrons exert degeneracy pressure; this creates a pulsar. In more massive stars, nothing can stop the collapse, which ends in the creation of a black hole.

Meteoroid
A meteoroid is a small, solid fragment of material in the solar system. An enormous number of these objects are present in the system. The term meteor is used to refer to such a body when it enters the Earth's atmosphere. Interaction (friction) between meteors and the upper levels of the atmosphere cause them to break up; most disintegrate before they reach the surface. The heat associated with frictional forces causes meteors to glow, creating the phenomena of shooting stars. The meteors that are large enough to avoid complete disintegration, and can therefore travel all the way down through the atmosphere to Earth's surface, are termed meteorites. Analysis of these fragments indicates that these bodies originate from the Moon, Mars, comets, and small asteroids that cross Earth's orbital path. The forceful impacts of meteorites on Earth's surface compress, heat, and vaporize some of the materials of the meteorite as well as crustal materials, producing gases and water vapor.


Asteroid

An asteroid is a small, solid planet (planetoid) that orbits the Sun. The orbital paths of most asteroids are between the orbits of Jupiter and Mars. Many of these bodies have been studied extensively and given names; those in the main belt (which tend to be carbonaceous) are classified into subgroups based on their distance from a large, named asteroid (for example, Floras, Hildas, Cybeles). Atens are asteroids whose orbits lie between the Earth and the Sun, and Apollos are asteroids with orbits that mimic Earth's. Asteroids may also be classified based on their composition. C-type asteroids exhibit compositions similar to that of the Sun and are fairly dark. S-type asteroids are made up of nickel-iron and iron- and magnesium-silicates; these are relatively bright. Bright asteroids made up exclusively of nickel-iron are classified as M-type.
Observation of the relative brightness of an asteroid allows astronomers to estimate its size.

Interstellar Medium
The interstellar, or interplanetary, medium (the space between planets and stars) is populated by comets, asteroids, and meteoroids. However, particles exist in this medium on an even smaller scale.
Tiny solid bodies (close to a millionth of a meter in diameter) are called interplanetary dust. The accumulation of this material in arctic lakes, for example, allows scientists to study it. Such analysis has revealed that these grains are most likely miniscule fragments of the nuclei of dead comets. They possess low density, for they are really many microscopic particles stuck together. The interplanetary dust refracts sunlight, which produces a visible (but faint) glow in parts of the sky populated by clouds of this dust. The interstellar medium also contains particle remnants of dead stars and gases (such as hydrogen molecules ionized by ultraviolet photons). Black holes (objects that collapse under their own gravitational forces), which trap photons, are also believed to populate the interstellar medium. Black holes are a form of dark matter.

Dark Matter
Observations of the gravitational force in the solar system (based on Kepler's laws) have indicated for years that there are bodies in the system that we cannot see. Dark matter (sometimes called missing matter) is thought to account for the unseen masses, though its exact nature is unknown. Some dark matter may simply be ordinary celestial bodies too small to be observed from Earth, even with technology such as high-powered telescopes. The presence of MACHOs (massive compact halo objects) has been noted through observation of distant galaxies—at certain times astronomers can discern dips in the brightness of these galaxies, thought to be caused by a large object (a MACHO) passing between Earth and the galaxy under observation.
Some have postulated that dark matter is made up of WIMPs (weakly interacting massive particles), which do not interact with photons or other forms of electromagnetic radiation; these particles are hypothetical, because astronomers cannot detect or study them.

Eclipses
Eclipses occur when one celestial body obscures the view of another, either partially or completely. A solar eclipse, or eclipse of the Sun by the Moon, happens when the Moon passes directly in front of the Sun (as observed from Earth).
Alternately, a lunar eclipse occurs when the Moon is situated in the Earth's shadow and is therefore completely invisible. These events do not happen every month because of the differential between the orbital planes of the Moon and the Earth—the Moon's orbit is about five degrees off from the ecliptic. The Moon's orbital path is subject to the same precession that occurs in the Earth's rotational axis; this causes the occasional intersection of the orbital planes of the two bodies. Therefore, eclipses are produced by a combination of the effects of the precession of the Moon's orbit, the orbit itself, and the Earth's orbit.

Newton's Law of Gravitation
Newton's law of gravitation (sometimes referred to as the law of universal gravitation) states that the force of gravity operates as an attractive force between all bodies in the universe. Prior to Newton's formulation of this law, scientists believed that two gravitational forces were at work in the universe—that gravity operated differently on Earth than it did in space. Newton's discovery served to unify these two conceptions of gravity.
This law is expressed as a mathematical formula: F = GMm/D2, in which F is the gravitational force, M and m are the masses of two bodies, D is the distance between them, and G is the gravitational constant (6.67 X 10–11).
The gravitational attraction between two objects, therefore, depends on the distance between them and their relative masses. Newton's law of gravitation served to clarify the mechanisms by which Kepler's laws operated. In effect, Newton proved Kepler's laws to be true through the development of this law.